The present invention relates to torque control for an induction motor in general, and more particularly to induction motor torque control system involving field-oriented control methods.
Field-oriented control is a substantial improvement over the V/f constant control method of the prior art which merely controls the voltage V and the frequency f supplied to the motor so as to hold both proportional to the speed of the motor, thereby to maintain the rotating flux constant. The main drawback of using voltage and frequency as the demand is that the dynamic control of the torque is not properly achieved because there is not a workable relationship between the torque as a variable and the voltage and frequency as the inputs.
In contrast thereto, field-oriented control methods do not call for voltage and frequency as control parameters. They are based on a reference frame which rotates at the speed of the rotor flux vector and upon which reference frame are oriented a flux component and a torque component of the stator currents which control the rotor flux and the resulting torque. These are coincident one with the d-axis, over the other with the q-axis. Typical of this approach is the control method shown in IFAC Symposium on Control in Power Electronics and Electrical Drives, Lausanne, Switzerland 1983, Pergamon Press, Oxford 1984: "Control of AC-Modules With The Help of Microelectronics," by W. Leonhard, pp 35-38. However, torque control in this prior art has been performed with current feedback control approach.
The present invention rests upon the idea with a field-oriented control method of no longer using current sensing as a current feedback control approach.